Gents,
I am building a 6AS7 based headphones amp with an LM338k regulated power supply. When I power it up, the heater voltage sags to just 1.2volts. I have checked everything and the unloaded supply reads well at 6.31 volts. But load with even one tube, and the supply simply sags to 1.2 volts. Does anyone have any idea what I could be looking at?

Cold heaters take a lot of current. This can send the regulator into over-current (or over-temperature) shutdown mode. Also check that the regulator input voltage does not sag under load and so violate the required in-out voltage.

I've been told to account for at least three times current at cold start, which takes you well over the 5Amps allowed by an LM338.

But it's quite likely to be input voltage dropping, as mentioned above.

You could try setting the regulator to say 8.8V and putting a 1 ohm resistor in series. This means your total resistance will be higher and your current lower over the warm up period. You'll need at least a 7W resistor suitably heat sunk. But then I don't know your reg's input voltage.

You could also try increasing the heat sink on the reg. As maybe it can cope with the brief over current when suitably cooled.

Thanks for the reply thus. Here's the circuit which I built. The transformer was specified at 50 VA. What I found last night is that this happens even when I just have the driver tube (6922) plugged in. I've got the power regulator on a generous heat sink and it does get quite warm.

Because I power up through a variac, would the ramping up cause a voltage violation issue with the LM 338K?

A slow ramp up might mean that the regulator is still struggling with insufficient voltage drop when it hits the current or temperature limit too. Better heatsinking might help, or add a resistor a Robert suggested.

A 50VA transformer will be able to continuously supply about 15-20W DC, so maybe 1.5A.

I have about 9.3 volts AC on the transformer primary. What concerns me is that even the 6922 by it self does not start. I had the wires twisted although I am using DC heaters. Would this be adding excessive capacitance to the power supply load?

I started just the 6922 heaters without any ramp up but it still refused to go higher then 1.24 or so volts.

Hi,
The max current for the LM338 it is 5 amps max. Your transformer can put out 50 VA /9 volts = 5 amps. Now the 6AS7 filament current is 2.5 amps. So your circuit it is within specs for the LM388. What are the size of the capacitors C10,C11 and C12?